With “Where Does My Tweet Go?” you can help understand how your tweets spread. And they told two friends, and so on, and so on.

Number to Know

2,996: The amount of people who are killed in the 9/11 attacks.

This Day in History

Sept. 11, 2001: Two hijacked aircraft crash into the World Trade Center in New York City, while a third smashes into The Pentagon in Arlington, Virginia, in a series of coordinated suicide attacks. A fourth hijacked airliner crashes in a field near Shanksville, Penn.